Many human diseases result from infection by microscopic organisms called viruses. Infection by viruses can give rise to symptoms that vary from mild to severe. Viral infections can result in large numbers of deaths. Examples of such pandemics include the Spanish flu of 1918-1919 that killed approximately 40 million people and the HIV/AIDS epidemic that has killed almost 2 million people.
Viruses require host organisms in order to replicate and viruses are transmitted from an infected host to an uninfected host through a number of mechanisms. A virus will first attach itself to a host cell. It will then enter the cell and release its genetic code (i.e., RNA or DNA). The virus makes use of the host cell's functional proteins and enzymes in order to replicate. Eventually, the host cell may die because the mechanisms it needs to survive are controlled by the virus. After death of the cell, the replicated viruses are released, allowing them to attack new host cells and continuing the replication process. Some viruses cause modification of the host cells leading to cancer, while other viruses can remain dormant in the host for an extended period prior to the infection becoming symptomatic in the host.
The symptoms that result from viral infections can vary from virus-to-virus as any one virus typically will infect only certain types of cells. This observation also means that a specific virus will typically infect only certain species, although mutation of a virus can allow it to extend the number of species that any one virus is able to infect.
Host species have developed a number of defense mechanisms to protect themselves from viral infections. The first lines of defense are mechanisms that prevent viral entry into the host. The skin provides an impermeable barrier to entry. Viruses typically enter the body through body cavities and can pass through the mucosal surfaces that line these cavities. Once a virus is in the body and detected by the body's immune system, lymphocytes and monocytes in the blood learn how to attack the invader. Invaded cells release cytokines such as the interferons (for example IL 1, IL 6, IL 12, IL 16), tumor necrosis factor (TNF-a), and interferons (typically interferons a and g). The role of these cytokines is to increase the resistance of other host cells to the invading virus. Many of the symptoms of viral infection experienced by the host results from the extensive release of cytokines, commonly referred to as the cytokine storm.
The white blood cells are able to remember how to combat viruses that have previously invaded the body. So if the host survives the initial attack of the virus, the immune system is able to respond much more quickly to subsequent infections of the same virus. The body has developed an immunity to the virus. Such immunity can also be induced by presenting the immune system with a surrogate (vaccine) for the virus in a process known as immunization.
Antiviral drugs are known in the art to assist the immune system in overcoming a viral infection in a patient. Most antiviral drugs work by slowing the replication of the virus in the infected patient's body thus allowing the body's immune system to launch an effective response when the disease symptoms are less severe. Antiviral drugs may work specifically on one or two viruses or may be effective across a broad spectrum of viruses. There are many known mechanisms by which antiviral agents can slow viral replication. One antiviral strategy is to slow or prevent the virus infiltrating a target cell, for example by binding to a receptor on the target cell which is required by the virus to enter the cell or by coating the virus so preventing its ability to bind to the target receptor(s). Other antiviral agents can slow viral replication once the virus particle has entered the target cell. Such mechanisms are well known in the art.